In steam turbines, the admission of steam to the inlet of the turbine is regulated by a series of control valves. To minimize throttling losses, such valves are sequentially opened and closed to regulate the flow of steam. Typically, such valves are mounted in a row along a steam chest which is in flow communication with the turbine inlet. Control valves are commonly hydraulically operated. However, since it is important to rapidly close the valves in the event of a turbine "trip", one or more springs are utilized to assist the hydraulic actuator to rapidly close the valve.
A typical steam turbine control valve 100 of the type previously known in the art is shown in FIGS. 1 and 2. The valve 100 is mounted via a bonnet 110 to a steam chest 111. A support post 107 extends upwardly from the bonnet 110 and terminates in a flange 101. A valve stem assembly, which includes the piston rod 113 of a hydraulic actuator 114 is disposed within the post 107. Inner and outer springs 108 and 109, respectively, surround the post 107 and are disposed between the flange 101 and a circular spring seat 102. The spring seat 102 has a circular hole 119 formed therein through which the post 107 extends, so that the spring seat is free to slide along the exterior of the post. A rectangular bar 104 is attached to the underside of the spring seat 102 by screws 105. The bar 104 extends through windows 106 formed in the post 107, so as not to interfere with the motion of the spring seat 102. The bar 104 rests on an annular washer 117 which rests on a seat 118 disposed on a coupling 116 which connects the piston rod 113 to the valve stem 115. Thus, when the hydraulic actuator force tending to open the valve is eliminated upon a turbine trip, the springs 108 and 109, acting through spring seat 102, bar 104 and coupling 116, thrust the valve stem 115 downward, thereby closing the valve 100.
During maintenance and assembly of the valve 100, the springs 108 and 109 must be restrained so that the spring force does not prevent the coupling 116 from being operated to connect and disconnect the piston rod 113 from the valve stem 115. Accordingly, three lugs 112 are formed on the periphery of the spring seat 102. When it is necessary to assemble or disassemble the coupling 116, bolts 103 are inserted through the flange 101 and lugs 112 so that the spring seat 102 can be secured to the flange.
Experience has shown that the control valve 100 shown in FIGS. 1 and 2 suffers from several disadvantages. First, providing the spring seat 102 with the capability of sliding over the post 107 by connecting the spring seat to the coupling 116 via a bar 104 which extends through the window 106 in the post results in a complex design with an excessive number of components.
Second, the relatively small diameter of the post 107 makes it fairly flexible. As a result, the valve 100 can vibrate excessively, resulting in fatigue cracking of the valve stem 115. One approach to increasing the stiffness of the control valve is to replace the support post 107 with a can-type spring housing which encloses the springs 108 and 109. The increased diameter of the spring housing relative to the support post 107 imparts increased stiffness to the valve. Such an arrangement is shown in U.S. Pat. Nos. 3,602,261 (Brown et al.) and 4,834,133 (LaCoste et al.) both of which are assigned to the same assignee as the current invention and which are hereby incorporated by reference.
According to the approach used in the prior art, the can-type spring housing was bolted to the valve bonnet by screws arranged in a circle around the spring housing. Since the screws attaching the bonnet to the steam chest were also arranged in a circle around the spring housing, the diameter of the spring housing had to be considerably less than the diameter of the bonnet. As a result, the space available on the bonnet to increase the diameter of the spring housing was not optimally utilized. Accordingly, it would be desirable to provide a method of attaching the bonnet to the steam chest and the spring housing to the bonnet which allowed the spring housing diameter to be as large as possible.
One disadvantage of the can-type spring housing is that, due to the fact that the can-type spring housing encloses the springs, the spring seat must slide inside of the housing. As a result, separate restraining bars, which project through the windows and bolt to the spring seat, must be used to restrain the spring seat during assembly and disassembly of the coupling. This results in a multiplication of the number of components and increases the man-hours required to assemble or disassemble the coupling.
Accordingly, it would be desirable to provide a control valve having considerable stiffness yet which required a minimum number of components and which allowed the spring seat to be readily restrained.